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Created by
 Western Graphics
Last modified
 08 February, 2006
 

AM2140 Module - Eµ Resonant Filter

Overview This module is a clone of the Resonant Filter in the Eµ Modular. The Resonant Filter is an audio filter with an output capable of producing virtually any two pole pass characteristic. The FREQUENCY control can be varied from 20Hz – 20kHz, and a resonant peak may be added in the response by means of the Q control. The height of the peak can be adjusted from zero to +40dB, which equates to a Q of 0 to 50.

The filter has three functions; Low Pass, BandPass and HighPass. The LowPass and HighPass have a 12dB slope, the BandPass has 6dB slope on both sides. The filter does not oscillate at high Q settings. An output mixer sums the simultaneous three filter functions to produce a single output, and three attenuators (HIGHPASS, BANDPASS and LOWPASS) mix the relevant signals. A wide range of responses can be produced and several 2140's can be cascaded to produce interesting complex resonant formants. This is an excellent sounding, warm filter, very useful in sound construction. Lower input levels should be used to prevent clipping at high Q levels. There is a signal input and output on the front panel, as well as an internal CV input on the board.

The 2140 is a basic filter with no external CV control of Q or FREQUENCY. There is a firmware CV for FREQUENCY on the board, which was typically connected to the 2145 Filter Controller which enabled multiple 2140's to be voltage controlled. The Resonant Filter of the Eµ Audity is very similar to the 2140, with the useful addition of voltage controlled Q.

Original Circuit Dave Rossum originally designed the Resonant Filter in 1973, and released the module in January 1974. The filter was updated in 1976 using the SSM2020 chip, and released as the Revision 2 design.

AM Circuit The AM circuit is a straight copy of the original Eµ Revision 2 design, but with upgraded Op Amps and an optional Q CV input. It uses the same SSM2020 chip but improved Op Amps for the main filter circuit. The mixing Op Amp was originally a 556, which is truly obsolete.

The frequency control uses a OP177 Op Amp to sum the front panel and a firmware CV input, a trimmer enables voltage control to be calibrated to 1V per octave. There is no temperature compensation. The Q control uses an exponential transistor circuit to drive a CA3080 OTA, as a VCA in the feedback signal. The original transistor pair was an AD820, which is expensive and hard to find. Provision has been made for a high quality SSM2220 pair on an 8 pin DIL, or manually matched transistor pairs, or the original AD820.

The SSM2020 chip is hard to locate, but they can still be found quite cheaply. The REV01 board of November 2002 was an initial test of the Eagle CAD program and PCB manufacturing (at least for me anyway!). A number of problems meant the board was never populated and tested. The REV04 as manufactured as a prototype in January 2004 and used as a test bed for the circuit. It worked very well but a small number of minor faults meant that the production board is REV05, and this was manufactured in January 2006.

Front Panel The front panel  is a standard AM design, 90mm wide and 4U high, with black lettering. Controls knobs are standard Eµ Modular designs.

 

* AM FAQ's more »
* Hard to find Parts more »
* Eµ Modular Catalog more »
 

* AM2140 Project Notes
 

* AM2140 PCB layout
   Zipped .BRD Eagle CAD
 

* AM2140 front panel
   Zipped .FPD Front Panel
   Designer file